1. Field of the Invention
The present invention generally relates to photoplotters and, more particularly, to alignment of exposure apertures in Gerber vector photoplotters.
2. Description of the Prior Art
Many modern manufacturing processes, particularly processes for fabricating electrical circuits, involve the production of intricate patterns of high accuracy. In particular, the production of masks for electronic and integrated circuits requires very accurate and complex patterns to be formed for the purpose of exposing photosensitive resists and the like.
For the creation of these patterns, the so-called Gerber vector photoplotter, manufactured by the Gerber Scientific Instrument Co. of South Windsor, Conn., has become widely used in the industry. Gerber vector photoplotters typically have an exposure head, an exposure table and a mechanism for causing an exposure head to move relative to the table with high speed and accuracy in two orthogonal directions. In more recent designs of such photoplotters, it is more usual to provide for movement of the table while holding the exposure head stationary to prevent the delicate optics of the head from being subjected to large acceleration loads. Thus, under computer control 30, the photoplotter can expose selected areas of a photosensitive medium at any location on the exposure table. Additional details of Gerber vector photoplotters may be found in U.S. Pat. Nos. 3,330,182 to Gerber et al, 3,724,347 to Gerber, 4,249,807 to Webster and 4,416,522 to Webster.
As particularly disclosed in U.S. Pat. No. 3,724,347 to Gerber, the exposure head of the photoplotter is typically divided into two sections. These two divisions are each optimized to produce the exposure of areas by either scanning the head over the area to be exposed, such as in line drawing, or by illuminating the entire area of a shape defined by a mask while the head is held stationary. This latter mode of operation is commonly referred to as flashing. Accordingly, these divisions are commonly referred to as a "line side" 32 and an "aperture 34 side" 34, respectively.
Since the pattern to be exposed will consist of geometric shapes, typically to form connection pads and lines to connect the pads, this division is for the purpose of producing the two types of images with greatest efficiency and accuracy. The pads can then be shaped in accordance with the shape of an aperture to avoid the need for movement of the head or table to develop the desired shape. Lines can be more efficiently exposed when the head or table is moved. As is known in the art, exposure profile across the width of a line can be affected by aperture shape; the dimension of the aperture in the direction of line drawing determining the effective exposure time (a circular aperture, for example, would produce reduced exposure at the edges of the line). Annular apertures for correction of the exposure across the width of lines are disclosed in greater detail in U.S. Pat. No. 3,548,713 to R. B. Webster. However, it is now common to use blade apertures in the line side of the photoplotter since the presently available photosensitive materials may easily be saturated during exposure, rendering the exposure profile relatively less important. Further, blade apertures allow for high speed adjustment of line widths under computer control as well as the capability of producing very fine lines.
Since these two portions of the exposure system in the exposure head must be used to form respective features of the same image, the geometrical relationship between the two must be accurately maintained. An offset must be developed to allow control of the positioning of the exposure head or table to exactly superimpose images formed by the two portions of the exposure head. For reasons of convenience, due to the structure of the aperture side, the aperture side is usually aligned to the line side of the photoplotter. The position (d) of the aperture on an aperture wheel which allows rapid changing of apertures will also affect the location of the aperture image and a correction (d) must be obtained for each aperture on the aperture wheel.
Apertures are typically positioned on the exposure wheel with an initial accuracy of about 1 mil, which is far less than the accuracy of which the Gerber vector photoplotter is capable. Since the aperture wheel extends into the optical system of the aperture side of the head, access is difficult for fine adjustment of aperture position on the aperture wheel. Therefore, it is customary to produce fine positional correction by adding or subtracting (e.g. at 36 of FIG. 3) orthogonal correction distances to a base compensation dimension 38 corresponding to the distance between the two sides of the exposure head. An arrangement for association of dimensional corrections as well as exposure controls with each aperture is disclosed in detail in U.S. Pat. No. 4,343,540 to Berdat.
In the past, these correction distances were determined by making a test exposure including a base line feature and an aperture image. Such a test exposure is made by exposing areas of a photosensitive medium, such as sheet film and developing the film to yield images for analysis. These images must be spaced by a small distance so that the centerline of each image can be optically determined by inspection of the image produced. For example, if the aperture image falls within or overlaps the base line image to any substantial degree, the centerline of the aperture image cannot be readily determined. The separation of the aperture and the base line images was typically on the order of 0.0001 inches to insure that no overlap occurred.
However, to exploit the accuracy available from the Gerber vector photoplotter, this separation of the centerlines of the images must be measured to a very high accuracy, on the order of 0.00002 inches, or about 1 part in 5 of the separation distance. While this ratio is as small as is considered practical, the separation distance itself introduces some uncertainty into the measurement. Further, such accuracy requires the use of large, stationary optical equipment and requires substantial time for the measurement to be made. When this time must be expended for two orthogonal directions for each of the many apertures of the photoplotter, the cost of operation of the photoplotter is greatly increased both in "down time" when apertures must be substituted and in labor costs of highly trained personnel to perform the measurement and enter the correction in the computer which controls the photoplotter. More importantly, perhaps, the measurement process is subject to error and an erroneous result may result in a defective pattern or even a run of defective manufactured parts.